A variety of vaccines have been developed for clinical use, mostly targeting the prevention of infectious diseases caused by viruses, bacteria and parasites. Vaccines can be prepared from live attenuated microbes, inactivated (killed) microbes, or components of the microbes themselves. Live attenuated microbes contain genetic alterations, such as deletion of virulence factors, resulting in a less virulent microbe. For inactivated vaccines, a microbe may be chemically or physically inactivated. Ideally, such vaccines cannot cause an infection but are still able to stimulate a desired immune response. Examples of inactivated vaccines include polio and influenza viruses, and bacterial vaccines against cholera and pertussis, although live attenuated vaccines are an option for polio, influenza, and cholera as well. In order to elicit the desired immune response, it is important that the inactivated microbe comprises the appropriate antigens prior to inactivation. It has been observed in some cases that inactivating the microbe results in a significantly reduced immune response because de novo gene expression by an infecting microbe is required to stimulate an optimal immune response. This is particularly important for intracellular bacteria. Methods that have been used to inactivate bacteria include the use of acetone, alcohol, formalin, glutaraldehyde, paraformaldehyde, or phenol, heating, or ultraviolet irradiation [Pace et al., Vaccine 16(16):1563 (1998)].
In addition to using microbial vaccines to prevent infectious diseases caused by the microbe itself, the microbes can be modified to contain heterologous nucleic acid sequences that encode a certain protein or antigen. Such recombinant microbes are used as delivery vehicles and may be used as vaccines to stimulate an immune response to the heterologous antigens. These recombinant vaccines have been shown to be effective in animal models. An oral vaccine of live attenuated Salmonella modified to express Plasmodium berghei circumsporozite antigen has been shown to protect mice against malaria [Aggarwal et al., J Exp Med 172(4):1083 (1990)]. Similarly, U.S. Pat. No. 6,051,237 describes a live recombinant form of Listeria monocytogenes that grows and spreads and expresses a tumor-specific antigen for use as a cancer vaccine. While such recombinant vaccines may be effective, each microbe strain must be genetically modified to provide the vaccine. It would therefore be desirable to develop a method of producing a safe and effective microbial vaccine that can be applied to any microbe, whether or not the microbe comprises recombinant antigens. Dendritic cell (DC)-based immunotherapy has been widely investigated and demonstrated to provide a clinical benefit for the treatment of a wide range of tumor types. A variety of strategies are presently being developed to isolate and generate autologous dendritic cells (DC), and subsequently load them with antigen or peptides ex vivo prior to patient vaccination. Recent advances in the understanding of immune mechanisms have, in addition to efficient antigen loading, highlighted the importance of the activation and maturation state of DC used for vaccination on the efficacy of cancer immunotherapy. Whereas immature DC are more effective in the uptake and processing of antigen, activated/mature DC lose this capacity, yet are more potent at presenting antigen to naïve T lymphocytes in the context of MHC molecules. In fact, mature DCs have been found to be potent antigen presenting cells (APC) to induce primary T lymphocyte responses, overcoming peripheral T cell tolerance and enhance anti-tumor immunity. Despite the development of a variety of methods to load and to stimulate the activation and maturation of DC that has led to encouraging clinical data, there still are not standard efficient and cost effective methods for combining antigen loading with DC activation and maturation.